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SIRQULATESIRQULATE

The science

What the research shows, including where it doesn't

Most brands publish the half of the literature that flatters them. This page has both halves, because the concessions are what make the rest worth reading.

The mechanism

How citrulline increases blood flowWhat the body does with citrulline

  1. 01

    L-citrulline is absorbed

    Taken by mouth, L-citrulline survives the first pass through the liver that breaks down most of an equivalent dose of arginine.

  2. 02

    The kidneys convert it to arginine

    Citrulline is converted into L-arginine, raising plasma arginine more effectively, gram for gram, than arginine taken directly.

  3. 03

    Arginine is the substrate for nitric oxide

    Arginine is what the body builds nitric oxide from — its own signalling molecule for relaxing the smooth muscle in blood-vessel walls. Substrate is not the same as output, and we don't claim it is.

  4. 04

    Vessels widen and blood flow rises

    Erection is, mechanistically, a nitric-oxide event — neurogenic NO is the principal signal for relaxing penile vessels. The same pathway is present in both bodies.This describes human physiology, not this product. Arousal is a circulatory event before it is anything else — in both bodies, by the same pathway.

Six capsules a day, split morning and evening, for a sustained effect rather than a single peak.Six capsules a day, split morning and evening — the way the pharmacokinetic study dosed it. Not an on-the-night product. A daily one.

The dose question

Is 3,000 mg enough?

If you are comparing it to a pre-workout: no, and we won't pretend otherwise. A premium scoop carries 5–6 g in one hit. This is a different job. The only controlled trial of citrulline on erection hardness used 1,500 mg a day — and this is twice that, every day, split morning and evening.If you are comparing it to a pre-workout: no, and we won't pretend otherwise. A premium scoop carries 5–6 g in one hit, taken once, for an acute effect. This is a daily split dose aimed at sustained plasma arginine, which is a different pharmacokinetic job — and the studies at this dose split it twice daily too.

What we will claim is purity. This is pure L-citrulline, not citrulline malate — there is no ratio to hide behind. Published NMR analysis of five UK “2:1” citrulline malates found four were nowhere near 2:1, and one delivered just over half the citrulline stated on the label.

Where we concede

Substrate is not the same as output

Citrulline reliably raises plasma arginine — at 1.5 g twice daily it does so more effectively than considerably larger doses of arginine taken directly, which is the reason the formula uses citrulline at all. What we will not tell you is that this translates into a measured rise in nitric oxide output at this dose, because in the study that measured it, the downstream markers moved at a higher dose than ours and not at this one.

The pump is not the mechanism people assume

The gym pump is largely not a nitric-oxide phenomenon — nitric oxide contributes something like 15% of the blood-flow response to exercise, and when researchers gave healthy subjects sildenafil and measured forearm flow during exercise, it made no difference at all. Erection, by contrast, is principally a nitric-oxide event. So the honest version of the fitness argument is the inverse of the intuitive one: this pathway has a better claim on intimate blood flow than it does on the pump.

In women, blood flow and feeling are only loosely related

Measured genital blood flow and felt desire correlate weakly in women — far more weakly than in men. And the largest trial of a PDE5 inhibitor in women, in 781 participants, failed on every patient-reported endpoint. Circulation is real physiology; it is not the whole of arousal, and the evidence says the strongest predictors of desire in women are relational rather than vascular.

What we are not going to cite

A widely repeated blood-pressure figure for citrulline that comes from a retracted analysis. A shilajit testosterone percentage taken from a small trial in men only. A claim that shilajit raises iron. And any statement that catuaba is a traditional aphrodisiac for women — the traditional use does not say that.